Patch "ring-buffer: Ensure proper resetting of atomic variables in ring_buffer_reset_online_cpus" has been added to the 5.10-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    ring-buffer: Ensure proper resetting of atomic variables in ring_buffer_reset_online_cpus

to the 5.10-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     ring-buffer-ensure-proper-resetting-of-atomic-variab.patch
and it can be found in the queue-5.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 5550b9391d989815469ac923e65bf8f87c2b477c
Author: Tze-nan Wu <Tze-nan.Wu@xxxxxxxxxxxx>
Date:   Wed Apr 26 14:20:23 2023 +0800

    ring-buffer: Ensure proper resetting of atomic variables in ring_buffer_reset_online_cpus
    
    [ Upstream commit 7c339fb4d8577792378136c15fde773cfb863cb8 ]
    
    In ring_buffer_reset_online_cpus, the buffer_size_kb write operation
    may permanently fail if the cpu_online_mask changes between two
    for_each_online_buffer_cpu loops. The number of increases and decreases
    on both cpu_buffer->resize_disabled and cpu_buffer->record_disabled may be
    inconsistent, causing some CPUs to have non-zero values for these atomic
    variables after the function returns.
    
    This issue can be reproduced by "echo 0 > trace" while hotplugging cpu.
    After reproducing success, we can find out buffer_size_kb will not be
    functional anymore.
    
    To prevent leaving 'resize_disabled' and 'record_disabled' non-zero after
    ring_buffer_reset_online_cpus returns, we ensure that each atomic variable
    has been set up before atomic_sub() to it.
    
    Link: https://lore.kernel.org/linux-trace-kernel/20230426062027.17451-1-Tze-nan.Wu@xxxxxxxxxxxx
    
    Cc: stable@xxxxxxxxxxxxxxx
    Cc: <mhiramat@xxxxxxxxxx>
    Cc: npiggin@xxxxxxxxx
    Fixes: b23d7a5f4a07 ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")
    Reviewed-by: Cheng-Jui Wang <cheng-jui.wang@xxxxxxxxxxxx>
    Signed-off-by: Tze-nan Wu <Tze-nan.Wu@xxxxxxxxxxxx>
    Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 1fe6b29366f10..f08904914166b 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -5051,6 +5051,9 @@ void ring_buffer_reset_cpu(struct trace_buffer *buffer, int cpu)
 }
 EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu);
 
+/* Flag to ensure proper resetting of atomic variables */
+#define RESET_BIT	(1 << 30)
+
 /**
  * ring_buffer_reset_cpu - reset a ring buffer per CPU buffer
  * @buffer: The ring buffer to reset a per cpu buffer of
@@ -5067,20 +5070,27 @@ void ring_buffer_reset_online_cpus(struct trace_buffer *buffer)
 	for_each_online_buffer_cpu(buffer, cpu) {
 		cpu_buffer = buffer->buffers[cpu];
 
-		atomic_inc(&cpu_buffer->resize_disabled);
+		atomic_add(RESET_BIT, &cpu_buffer->resize_disabled);
 		atomic_inc(&cpu_buffer->record_disabled);
 	}
 
 	/* Make sure all commits have finished */
 	synchronize_rcu();
 
-	for_each_online_buffer_cpu(buffer, cpu) {
+	for_each_buffer_cpu(buffer, cpu) {
 		cpu_buffer = buffer->buffers[cpu];
 
+		/*
+		 * If a CPU came online during the synchronize_rcu(), then
+		 * ignore it.
+		 */
+		if (!(atomic_read(&cpu_buffer->resize_disabled) & RESET_BIT))
+			continue;
+
 		reset_disabled_cpu_buffer(cpu_buffer);
 
 		atomic_dec(&cpu_buffer->record_disabled);
-		atomic_dec(&cpu_buffer->resize_disabled);
+		atomic_sub(RESET_BIT, &cpu_buffer->resize_disabled);
 	}
 
 	mutex_unlock(&buffer->mutex);